While the hereinafter described invention was conceived for use in reducing the noise in the cabin of an aircraft, it is to be understood that the invention can be used in other environments and reinforced skin structures to reduce interior noise and vibration. This includes all types of transportation vehicles--automobiles, buses, trucks, ships, submarines, hovercraft and hydrofoils, for examples. The invention can also be used in the exterior and interior walls of buildings and enclosures where high noise transmission reduction is desired. The invention can also be used on reinforced skin bulkheads, partitions or walls in any or all of the transportation vehicles listed above and others, including aircraft.
It is also to be understood that, because the interior noise is reduced by damping the vibrations of a reinforced skin structure, coincidental to the reduction of noise is a corresponding improvement in the sonic fatigue life of the structure and equipment attached to the structure. That is, reducing vibrations not only reduces noise, it also improves the sonic fatigue life of the vibrating structure and attached equipment.
Noise and vibration inside of a reinforced skin structure, such as the cabin of an aircraft, affects passenger speech communication, comfort and sleep. Noise and vibration also can cause fatigue and, thus, the malfunction of equipment mounted in regions of high noise and vibration. Since most transportation structures are designed to be as light in weight as possible (commensurate with structural requirements), in order to obtain maximum fuel efficiency, limitations are placed on what designers can do to reduce interior noise and vibration levels. These constraints are particlarly severe in the aircraft design field where weight is extremely critical.
In general, noise in an aircraft can be segregated into noise contributing to the overall sound pressure level (OASPL) and noise contributing to the speech interference level (SIL). The OASPL is essentially determined by the low frequency content of the noise and SIL is determined by the mid to high frequency content of the noise. Since both the OASPL and SIL affect passengers, a noise reduction over the entire audio frequency range and, in particular, the low and mid-audio frequency range is desirable. Correspondingly, the whole frequency range is an important contributor to noise and vibration induced fatigue and malfunctioning of equipment. In this regard, even though the following discussion centers on the reduction of interior noise for passenger comfort, the invention is equally applicable to reducing the detrimental effects of noise and vibration on equipment and structure.
Presently the interior or cabin noise of an aircraft in the mid and high frequency range (above 600 Hz) is reduced by applying skin damping tape, lead vinyl sheeting and fiberglass insulation to the walls of the aircraft fuselage. While the use of such items to reduce noise are effective in the mid and high frequency range, they are essentially ineffective in the low frequency range, particularly at frequencies below 300 Hz. Further, they are only moderately effective in the mid-frequency range between 300 and 600 Hz. As a result, the reduction of low and mid-frequency cabin noise has remained a problem in present commercial aircraft.
While, both low and mid-frequency cabin noise remains a problem in present aircraft, the problem is acute in recently developed short takeoff and landing (STOL) aircraft, such as externally blown flap (EBF) and upper surface blown (USB) aircraft. The problem is acute in such STOL aircraft because the level of low frequency interior noise is higher due to the proximity of the engines to the fuselage of the aircraft. As a result, it has now become even more desirable to provide improved methods and apparatus for reducing the OASPL and the SIL in the cabin of aircraft.
In the past, it was generally believed that cabin noise below about 600 Hz was controlled by the structural stiffness of the fuselage of the aircraft. Thus, attempts to reduce low and mid-frequency cabin noise were based on various methods of increasing fuselage structural stiffness. For example, in one attempt, the number of stringers in the fuselage of a modern aircraft were doubled to increase the structural stiffness of the fuselage and, thereby, reduce cabin noise. Test data taken on this aircraft indicated that although this 100 percent increase in stringer weight was partially effective in reducing cabin noise in the mid-frequency range (e.g., 300-600 Hz), it was ineffective in the low frequency range (e.g., below 300 Hz). Thus, although this change improved the subjective impression of the noise level in the cabin of the aircraft, the overall sound pressure level (OASPL) was virtually uneffected.
In recent years, it has been found that during cruise, when the pressurization loads cause the skin panel frequency of an aircraft to be higher than the stringer frequency, the coupled mode of the overall reinforced skin structure is such that the skin acts like a very stiff member, supported by relatively flexible stringers. In this regard, attention is directed to U.S. Pat. No. 3,976,269 entitled "Intrinsically Tuned Structural Panel" by Gautam SenGupta. This coupled mode is a very strong radiator of sound because a large section of the skin vibrates in phase. That is, the individual sections of the skin vibrate in phase, whereby vibrations combine to form noise sources having a relatively high magnitude. Since the skin responds like a very stiff member, very little skin flexural bending takes place. As a result, the application of damping devices (e.g., damping tape) to the skin is not very effective in reducing the low frequency noise produced by such structures. On the other hand, the vibration response of this coupled mode is strongly determined by the deflection of the relatively flexible stringers. As a result, damping the stringers is a very effective way of reducing the low frequency response of the overall structure.
A method and apparatus for significantly reducing the noise produced by stringer response is described in U.S. patent application, Ser. No. 029,705, entitled "Method and Apparatus for Reducing Low to Mid-Frequency Interior Noise," filed Apr. 11, 1979, by Gautam SenGupta and Byron R. Spain. This patent application describes reducing stringer response to vibration disturbances by applying rigid strips across the stringer flanges, the rigid strips being attached to the flanges by thin viscoelastic layers. This method of stringer damping has been found to reduce structural vibration and cabin noise during cruise in the low frequency range.
While stringer damping using the method and apparatus described in the foregoing patent application is effective in reducing noise when stringer vibration is the dominant noise source, when skin vibration is the dominant noise source, this method is ineffective. In this regard, during takeoff skin vibration is the dominant noise source in most presently designed aircraft. In order to overcome this problem, the foregoing patent application teaches forming the aircraft fuselage such that the fundamental frequency of the skin is higher than the fundamental frequency of the skin supporting stringers. However, unless this is achieved through cabin pressurization, this approach can lead to an increase in the weight of the aircraft. Alternatively, separate devices can be used to damp skin vibrations. The separate devices must cover substantially the entire skin area and, thus, add a substantial amount of weight.
In summary, prior to the present invention, reinforced structures, such as the reinforced skin structures forming the fuselage of an aircraft, have had one type of dampling applied to the reinforcing members, e.g., stringers and frame members, and another type of damping applied to the skin. In this manner, whichever element is dominant with respect to vibrational response is damped. However, the damping treatments do not cooperate such that each damping treatment assists the other damping treatments when the other damped structural element creates the dominant response to a vibrational disturbance. Because of a lack of cooperation, the total amount of weight added is higher than desired. In this regard, it should be noted that in order for prior art skin damping to be effective it had to be applied to large areas of the skin. The use of damping materials in isolated regions, such as the center of skin panels was ineffective in reducing noise. Alternatively, it has been proposed to form the structure such that reinforcing member vibration is dominant; and, to damp the vibrations of the reinforcing member. This approach has the disadvantage of increasing the weight of the structure as a result of the additional reinforcing members needed to produce the necessary skin stiffness.
Therefore, it is an object of this invention to provide a new and improved method and apparatus for damping the vibrational response of reinforced structures.
It is also an object of this invention to provide a new and improved method and apparatus for damping the vibrational response of reinforced skin structures.
It is another object of this invention to provide a wideband cabin noise and vibration reduction method and apparatus for use with a reinforced skin structure.
It is a still further object of this invention to provide a wideband reinforced skin structure vibration damping method and apparatus that functions to damp the vibrational response of both the skin and the skin reinforcing members of a reinforced skin structure.